This invention relates to monitoring vital signs in humans and animals, in particular one or more of heart rate, respiration rate and temperature.
Conventionally, heart or pulse rates are determined through measurements of the electrical activity of the heart. For example, an electrocardiograph (ECG) uses two or more electrodes attached about the chest to measure the electrical activity of the heart and hence determine heart rate. In a hospital environment, electrocardiographs offer an accurate measurement of heart rate as well as detailed information about the activity of the heart muscles. More basic monitors of heart rate activity are available, such as the type of heart rate monitor worn by athletes which typically determine heart rate but do not provide further information of the type which can be useful to diagnose heart problems. Heart rate monitors for sports typically include a chest strap comprising the electrodes for measuring the electrical activity of the heart and a device to which the measurements are transmitted for processing into a heart rate value, for example a watch or smartphone. Such chest straps can be uncomfortable to wear and for reliable operation require the user to sweat or prepare the electrodes of the chest strap with a conductive gel to electrically couple the electrodes to the skin.
Respiration rate can be measured in various ways, although it is often not directly measured in a clinical setting (measurements of oxygen saturation in the blood is typically of more immediate value). Various kinds of pneumographic apparatus is available for directly measuring the rate of respiration which have different principles of operation. This includes apparatus which measures the expansion and contraction of the chest through the use of a flexible strap attached about the chest, and apparatus which measures the changes in electrical resistance across the chest cavity as the user's lungs expand and contract. Other types of apparatus directly measure the inhalation and exhalation of air through a patient's mouth or breathing tube.
Pneumographic apparatus is generally bulky and designed for use in a hospital or sports science environment.
Various applications are available for smartphones which utilise the accelerometer present in smartphones to determine the respiratory rate of a user when the smartphone is placed on the chest of a user lying in a supine position. It has also been established by Kwon et al. in Validation of heart rate extraction through an iPhone accelerometer, Conference Proceedings of the IEEE Engineering in Medicine and Biology Society 2011; pp. 5260-5263, that the accelerometer of an iPhone can be used to capture an estimate of heart rate by strapping the phone to the chest. It is however uncomfortable to wear a smartphone in this manner for long periods of time and the power demands of operating a smartphone in this manner do not allow its battery to support such an operation for more than a few hours. Smartphone applications are also available which capture a rough estimate of heart rate at a moment in time using the smartphone's camera.
Neither the conventional apparatus used in hospitals to measure heart and respiration rate, nor the apps available on smartphones, allow the vital signs of a user to be monitored for extended periods of time as they go about their daily activities.